INHIBITION OF O-2-REDUCING ACTIVITY OF HORSERADISH-PEROXIDASE BY DIPHENYLENEIODONIUM

Plant cells respond to pathogen attack with a burst of H2O2 secretion. The question whether this defense reaction is catalysed by a NAD(P)H oxidase similar to the NADPH oxidase of phagocytic leukocytes in mamma ls or by an extracellular peroxidase is presently a matter of controve rsial debate. The observation that H2O2 production by plant cells can be inhibited by diphenyleneiodonium (DPI), a potent inhibitor of the m ammalian NADPH oxidase, has fostered the view that a mammalian-type en zyme is responsible for the H2O2 production by plant cells. Here we sh ow that DPI inhibits the NADH-dependent H2O2 production by horseradish peroxidase in the same concentration range as previously used for the inhibition of putative NADPH oxidase activity in plants. The peroxida tive activity normally used for assaying peroxidase is not affected by DPI, indicating that the inhibitor specifically interferes with a par tial reaction that is exclusively involved in the O-2 reducing activit y of the enzyme. (C) 1998 Elsevier Science Ltd. All rights reserved.